The present invention relates to a novel process for making alkali metal and alkaline earth metal salts of L-ascorbic acid 2-monophosphate starting from a L-ascorbic acid salt, a L-ascorbic acid 2-polyphosphate and an alkaline earth metal hydroxide as the base.
As is known, ascorbic acid (vitamin C) and its salts are used as additives for human and animal foodstuffs. However, ascorbic acid itself is temperature-and oxidation-sensitive and is decomposed to a considerable extent, for example, in the production and storage of enriched fish feed and is thus lost. The ascorbic acid phosphates in particular are, as is known, forms of ascorbic acid which are protected against oxidative and thermal degradation and are accordingly and primarily used for the nutritional enrichment of fish feed. With the use of ascorbic acid phosphates, which are substantially more stable than ascorbic acid, the problem of decomposition is almost completely eliminated and the ascorbic acid, which is active, for example, against scurvy in fish and crabs, is liberated in the host organism by the action of the enzyme phosphatase.
Two fundamentally different processes have hither to been of significance for the phosphorylation of ascorbic acid, namely phosphorylation using phosphorus oxychloride (as described, for example, in European Patent Publications 388,869 and 582,924 as well as in U.S. Pat. No. 4,179,445) and phosphorylation using polyphosphates, e.g., sodium trimetaphosphate (see, for example, U.S. Pat. Nos. 4,647,672 and 5,110,950), with a L-ascorbic acid salt being phosphorylated under basic conditions in both cases. The first process yields ascorbic acid 2-monophosphate as the main product and as byproducts mainly ascorbic acid 3-phosphate and 2-pyrophosphate as well as bis(ascorbic acid)-2,2'-diphosphate [see C. H. Lee et al., Carbohydrate Res. 67, 127-138 (1971)]. The reaction products require complicated purification and cannot be converted in a simple manner, e.g., by spray drying of the entire reaction mixture, into a product which can be commercialised directly. For these reasons phosphorylation using phosphorus oxychloride is a process which has little attraction economically and ecologically. The alternative process, i.e. phosphorylation using polyphosphates, yields ascorbic acid 2-polyphosphate as the primary product, for example ascorbic acid 2-triphosphate when sodium trimetaphosphate is used. The ascorbic acid 2-polyphosphates can be degraded to the monophosphate by an excess of base. The ratio of ascorbic acid 2-monophosphate to ascorbic acid 2-diphosphate and higher phosphates is influenced by the amount of base which is used and the other reaction conditions. A disadvantage of this process is that a very large amount of phosphorylating agent, e.g., at least 1 mol of sodium trimetaphosphate per mol of ascorbic acid, is required. Moreover, when the product should contain relatively little ascorbic acid polyphosphates and more ascorbic acid 2-monophosphate, a large amount of base, e.g., calcium hydroxide, must be used. Accordingly, the product contains a large amount of inorganic phosphates; the content of ascorbic acid equivalents in a dried product amounts to a maximum of about 25 weight percent. The course of the process described in U.S. Pat. No. 5,110,950, insofar as it is used for the manufacture of the monophosphate, will be evident from Reaction Scheme 1 hereinafter in which, for the purpose of a clear and simple presentation, sodium ascorbate, sodium trimetaphosphate (the preferred phosphorylating agent) and calcium hydroxide (base) are used: ##STR1##
In this process sodium ascorbate is reacted with sodium trimetaphosphate in a molar ratio of 1:1 under the influence of a total of 2.5 mol of calcium hydroxide in order to produce 1 mol of ascorbic acid 2-monophosphate after three steps. Thereby, 2 mol of sodium calcium phosphate are liberated, the presence of which in the overall product is seen to be a disadvantage.